1. Field of the Invention
The present invention relates to a radiation image detecting device having a line defect correction function for correcting a strip-shaped line defect occurring in a radiographic image, an operating method of the radiation image detecting device, and a radiation imaging system.
2. Description Related to the Prior Art
In a medical field, an X-ray imaging system using X-rays, as a kind of radiation, is known. The X-ray imaging system is constituted of an X-ray generating apparatus for generating the X-rays, and an X-ray imaging apparatus for taking an X-ray image of an object (a patient) by receiving the X-rays passed through the object. The X-ray generating apparatus includes an X-ray source for emitting the X-rays to the object, a source control unit for controlling the operation of the X-ray source, and an emission switch for inputting a command to actuate the X-ray source to the source control unit. The X-ray imaging apparatus includes an X-ray image detecting device for detecting the X-ray image that is produced from the X-rays passed through the object, and a console for controlling the operation of the X-ray image detecting device and storing and displaying the X-ray image.
A kind of the X-ray image detecting device using an image detector (a flat panel detector, FPD) that detects the X-ray image as an electric signal has become widespread. The image detector is constituted of a panel unit and a circuit unit. The panel unit has an image capturing field for capturing a radiographic image of the object. The panel unit has a plurality of pixels arranged in two dimensions and signal lines. The pixels each for accumulating electric charge produced in accordance with an X-ray amount incident thereon are arranged in a plurality of pixel-rows and pixel-columns. The electric charge is read out from the pixels through the signal lines on a pixel-row basis. Each pixel is provided with a photoelectric conversion element for producing and accumulating the electric charge, and a switching element such as a TFT (thin film transistor). The circuit unit includes a gate driver, a signal processing circuit, and a controller for controlling the operation of the panel unit through the gate driver and the signal processing circuit.
The gate driver issues gate pulses to drive the switching elements through scan lines provided on a pixel-row basis. The signal processing circuit outputs voltage in accordance with the electric charge read out through the signal lines provided on a pixel-column basis. The controller makes the panel unit perform three operations, that is, a pixel reset operation for discharging the electric charge accumulated in the pixels, an accumulation operation for accumulating the electric charge in the pixels by turning off the switching element of every pixel, and an image readout operation for reading out the electric charge from the first pixel-row to the last pixel-row on a pixel-row basis after the completion of the accumulation operation to capture an X-ray image of one frame (one screen) to a frame memory.
The pixel reset operation is an operation for removing dark charge accumulated in the pixels. The dark charge, which is based on dark current, becomes a noise component of the X-ray image, and hence the pixel reset operation is carried out. In the pixel reset operation, pixels are sequentially reset from the first pixel-row to the last pixel-row on a pixel-row basis, and after completing the reset of the last pixel-row, the reset is repeated again from the first pixel-row.
In order to appropriately take the X-ray image, the X-ray imaging apparatus requires information on the start of X-ray emission from the X-ray source. Japanese Patent Laid-Open Publication No. 2011-254971 discloses a communication method in which a signal related to the X-ray emission is communicated between the X-ray generating apparatus and the X-ray imaging apparatus, and a self-judgment method in which the X-ray image detecting device judges the start of the X-ray emission.
In the communication method, the X-ray image detecting device performs the pixel reset operation before the start of the X-ray emission. In the pixel reset operation, the electric charge accumulated in the pixels is read out and discharged on a pixel-row basis. If an emission start request is sent from the X-ray generating apparatus to the X-ray image detecting device during the pixel reset operation, the X-ray image detecting device permits the X-ray generating apparatus to start the X-ray emission at the time of completing the reset of the pixels of the last row. After the permission of the start of the X-ray emission, the X-ray image detecting device is shifted from the pixel reset operation to the accumulation operation.
In the self-judgment method, on the other hand, the start of the X-ray emission is judged by detecting variation of the signal due to the X-rays during the operation of the X-ray image detecting device. The X-ray image detecting device performs the accumulation operation between two types of readout operations, that is, a pre-emission readout operation for reading out the electric charge in predetermined cycles on a pixel-row basis and the image readout operation. If the X-ray emission is started during the pre-emission readout operation, the electric charge of each pixel rapidly increases and the signal varies largely, and thereby the start of the X-ray emission is detected. Upon detecting the start of the X-ray emission, the pre-emission readout operation is stopped and shifted to the accumulation operation. After a lapse of predetermined emission time, the image readout operation is carried out to sequentially read out the electric charge of the pixels from the first row on a pixel-row basis and produce the X-ray image based on the X-ray emission.
In the communication method, the accumulation operation is carried out and no electric charge is read out during the X-ray emission. According to the self-judgment method, on the other hand, in the duration between the start of the X-ray emission and the judgment of the emission start, the electric charge of a plurality of pixel-rows is read out by the pre-emission readout operation. Thus, not only the dark charge but also the electric charge produced by the X-ray emission is read out from the plurality of pixel-rows. This electric charge that is readout in advance is turned to be a deficit, and therefore a line of low density i.e. a line defect appears in the X-ray image.
According to the X-ray image detecting device described in the Japanese Patent Laid-Open Publication No. 2011-254971, the line defect is corrected based on a reference line image in which output of the electric charge read out sequentially in the pre-emission readout operation is recorded on a pixel-row basis. This X-ray image detecting device uses the reference line image for the judgment of the emission start too.
To be more specific, as shown in FIG. 18, in a standby state before the X-ray emission, the pre-emission readout operation is performed in which the gate driver sequentially issues gate pulses G(1) to G(N) (N is the number of the pixel-rows) at predetermined intervals H to read out the electric charge from the pixels of the first pixel-row to the last pixel-row on a pixel-row basis. Upon completing the reset of the last pixel-row, the readout is repeated from the first pixel-row. The output of the electric charge that is read out of the one pixel-row is recorded to the frame memory, as the reference line image.
By completing the readout of one frame, a reference frame image RP, which is composed of the reference line images of one frame, is recorded. A readout period of one frame is defined as one cycle. Upon completing the one cycle, the next cycle is started and the readout is repeated from the first pixel-row. Concurrently, the reference frame image RP is updated on a pixel-row basis using the reference line images of the next cycle.
In the judgment of the emission start, as shown in FIG. 19, a typical value of pixel values S of the reference line image sequentially outputted at the intervals H is compared with a predetermined judgment threshold value Th. As the typical value of the pixel values S of the reference line image to be compared with the judgment threshold value Th, a maximum value of the pixel values S of one pixel-row, or an average value or a sum value of the pixel values S of one pixel-row is used. As shown in an X-ray emission profile, which represents time-variation in an X-ray dose applied from the X-ray source per unit of time, the X-ray dose applied per unit of time is low immediately after the start of the X-ray emission, and is gradually increased to a set dose value determined in accordance with tube current. Round marks indicated with “E” represent the output timing of the reference line images. Letters “C”, “C−1”, and the like represent the pixel-rows from which the reference line images are outputted.
Before the start of the X-ray emission from the X-ray source, the pixel values S of the reference line image correspond to output according to the dark charge, and are much smaller than output according to the X-ray dose. Thus, the pixel value S according to the dark charge is regarded as a level of approximately zero in FIG. 19. After the start of the X-ray emission, the pixel values S of the reference line image are increased in accordance with the X-ray emission profile. After that, the typical value of the pixel values S exceeds the judgment threshold value Th. At the instant when the typical value of the pixel values S of the reference line image exceeds the judgment threshold value Th, the X-ray emission from the X-ray source is judged to be started.
As shown in FIG. 18, as soon as the X-ray emission is judged to be started, the controller immediately stops issuing the gate pulses and shifts the panel unit from the pre-emission readout operation to the accumulation operation. After a lapse of time determined in an imaging condition, the X-ray emission is expected to be completed, and the panel unit shifts to the image readout operation. The electric charge is read out on a pixel-row basis and an X-ray image XP is outputted. After the image readout operation, the panel unit shifts to the pre-emission readout operation again in a case where a reservation for the next imaging is made. The panel unit completes its operation in the case of no reservation for the next imaging.
FIGS. 18 and 19 show a state in which the typical value of the pixel values S of the reference line image of the Cth pixel-row exceeds the judgment threshold value Th, and hence the judgment of the emission start is made at the Cth pixel-row. However, the X-ray emission is actually started just moments before reading out the reference line image of the (C−2)th pixel-row, which is two pixel-rows previous to the Cth pixel-row (just moments before inputting the gate pulse (C−2) to the pixels of the (C−2)th pixel-row). Such a time delay between the start of the X-ray emission and the judgment of the emission start causes the readout of three pixel-rows, including the Cth pixel-row immediately before the stop of the pre-emission readout operation and the (C−1)th and (C−2)th pixel-rows being next previous to the Cth pixel-row, during the X-ray emission. This brings about the deficit in the electric charge.
Such a deficit in the electric charge manifests itself as a strip-shaped line defect extending in a pixel-row direction (X direction) in the X-ray image XP, as shown in FIG. 20. In the drawing, a right graph shows a plot of pixel values D of an arbitrary column X (X=1 to M, M is the number of the pixel-columns) in the X-ray image XP along a pixel-column direction (Y direction). The pixel value D begins decreasing from the (C−2) th pixel-row at which the pre-emission readout operation is performed just moments after the start of the X-ray emission, and comes to the lowest at the Cth pixel-row at which the judgment of the emission start is made and just moments before stopping the pre-emission readout operation. Accordingly, the line defect becomes severer in a stepwise manner from the (C−2)th pixel-row to the Cth pixel-row, and the severest at the Cth pixel-row. A difference in density is conspicuous between the Cth pixel-row and the next (C+1)th pixel-row, due to a difference in the pixel values D. Note that, the plot does not show an effect of attenuation of the X-rays by the object and an offset of the dark charge (the same goes for FIG. 21).
As shown in FIG. 21, the reference frame image RP represents the offset caused by the dark charge of the pixels before the X-ray emission. However, the reference line images of the three pixel-rows, i.e. from the (C−2)th pixel-row at which the pre-emission readout operation is firstly performed after the start of the X-ray emission to the Cth pixel-row at which the judgment of the emission start is made and just moments before the stop of the pre-emission readout operation, represent output according to the X-ray dose and correspond to the line defect in the X-ray image XP.
The pixel values S of the reference line images of the three pixel-rows corresponding to the line defect are increased in a stepwise manner from the (C−2)th pixel-row to the Cth pixel-row, oppositely to the line defect in the X-ray image XP. In FIG. 21, a right graph shows a plot of pixel values S of the arbitrary pixel-column X in the reference frame image RP along the Y direction, just as with the plot of FIG. 20. The pixel values S of the reference frame image RP are approximately zero from the first pixel-row to the (C−3)th pixel-row before the start of the X-ray emission. The pixel value S begins increasing from the (C−2)th pixel-row, and comes to the highest at the Cth pixel-row. The typical value of the pixel values S exceeds the judgment threshold value Th at the Cth pixel-row. The pixel values S of the (C+1)th pixel-row to the Nth pixel-row are obtained in the pre-emission readout operation of the previous cycle, and are approximately zero just as with the pixel values S of the first to (C−3)th pixel-rows.
The difference in the pixel value D of the X-ray image XP between the Cth pixel-row just moments before stopping the pre-emission readout operation and the next (C+1)th pixel-row becomes maximum, as for the difference in the pixel value D between adjoining two pixel-rows of the pixel-rows corresponding to the line defect of the X-ray image XP. This difference is called a difference amount. Representing an absolute value of the difference amount as AD (see FIG. 20), the difference amount AD is equal to a pixel value SQ (see FIG. 21) of the Cth pixel-row in the reference frame image RP. Furthermore, a falling gradient of the pixel value D from the (C−2) th pixel-row to the Cth pixel-row in the X-ray image XP coincides with a rising gradient of the pixel value S from the (C−2)th pixel-row to the Cth pixel-row in the reference frame image RP. In other words, the complementary relation holds between the pixel values D of the pixel-rows having the line defect in the X-ray image XP and the pixel values S of the pixel-rows corresponding to the line defect in the reference frame image RP. Therefore, in the Japanese Patent Laid-Open Publication No. 2011-254971, the reference frame image RP is used as a correction image for correcting the line defect. The line defect of the X-ray image XP is corrected by adding the pixel values S of the pixel-rows corresponding to the line defect in the reference frame image RP to the pixel values D of the pixel-rows having the line defect in the X-ray image XP.
By the way, according to another method of the readout operation, gate pulses are concurrently inputted to a plurality of adjoining pixel-rows and the electric charge accumulated in the pixels is discharged from the plurality of pixel-rows at a time to the signal lines, in order to shorten time required for the one cycle. In reading out the electric charge from the plurality of adjoining pixel-rows at a time, the electric charge of the pixels of the plural pixel-rows is added in each signal line on a pixel-column basis. Such a readout operation by which the electric charge is readout from the plurality of pixel-rows in a state of addition is called binning readout. Also, the plurality of adjoining pixel-rows from which the electric charge is read out at a time is called a binning pixel-row. The binning readout can shorten time required for the one cycle of the readout from the first row to the last row, as compared with the case of the readout on a pixel-row basis, and hence facilitates reducing the dark charge accumulated in the pixels.
However, in the case of performing the binning readout as the pre-emission readout operation, there is a problem that the line defect of the X-ray image XP cannot be corrected in the method described above, that is, by simply adding the pixel values S of the reference frame image RP to the pixel values D of the X-ray image XP.
This is because in the binning readout, the electric charge is discharged at a time from the pixels of the plurality of pixel-rows composing one binning pixel-row. Thus, the reference line image is composed of values each of which corresponds to the electric charge of the plurality of pixels, for example, four pixels, outputted in a state of being added on a column-by-column basis. Sequentially recording the reference line images on a binning pixel-row basis allows obtainment of the reference frame image RP. On the other hand, the X-ray image XP is read out on a pixel-row basis. Thus, the pixel value SQ of the reference frame image RP recorded by the binning readout at a part corresponding to the line defect is larger than the difference amount ΔD in the X-ray image XP at a part of the line defect, and the values do not coincide each other. In the reference frame image RP recorded by the binning readout, since the one binning pixel-row is composed of the plurality of pixel-rows, the number of the binning pixel-rows of the reference frame image RP is less than the number of the pixel-rows of the X-ray image XP. Setting the four pixel-rows as the one binning pixel-row, for example, the number of the binning pixel-rows of the reference frame image RP is a quarter of the number of the pixel-rows of the X-ray image XP. Thus, the reference frame image RP recorded in the binning readout is of the same image size as the X-ray image XP in the pixel-row direction, and of smaller image size in the pixel-column direction. Therefore, the complementary relation between the pixel values D of the pixel-rows having the line defect in the X-ray image XP and the pixel values S of the pixel-rows corresponding to the line defect of the reference frame image RP does not hold true. Therefore, the line defect of the X-ray image XP cannot be corrected by simply adding the reference frame image RP to the X-ray image XP, as described in the Japanese Patent Laid-Open Publication No. 2011-254971.